1. Field of the Invention
This invention relates to a thin film transistor device with a lightly doped drain (called xe2x80x9cLDDxe2x80x9d) structure, a method of manufacturing the same and a thin film transistor liquid crystal display device.
2. Discussion of the Background
A thin film transistor (called xe2x80x9ca TFTxe2x80x9d) made of a polycrystalline silicon semiconductor layer (collectively called xe2x80x9ca p-Si TFTxe2x80x9d) is now researched and developed in order to comply with requirements for the integration of driver circuits with a p-Si TFT array formed on a glass substrate of a liquid crystal display device.
It is well known that, similarly to a single crystalline silicon TFT, a breakdown or deterioration of the TFT characteristics takes place with a p-Si TFT when an excessively strong electric field is applied between the source and drain regions thereof. Such a phenomenon is mainly caused by the occurrence of hot carriers due to concentration of the electric field on a certain location in the vicinity of the drain region so that it becomes a more critical problem as the TFT becomes much finer in structure.
In order to solve those technical problems a lightly doped drain structure is effective as often used in a single crystalline TFT. Since, however, anisotropic etching, low temperature processes and so on are required for devices formed on a large glass plate, such as those used for a liquid crystal display, the single crystalline processes are not always directly applicable to the polycrystalline processes.
There have been proposed process techniques for forming an LDD structure TFT which are different from those for single crystalline processes. A side wall forming technique, for example, by means of anodic oxidation of the gate electrode is disclosed in Japanese Unexamined Patent Publication Hei 6-104279. A method of making use of side etching at the gate electrode is also disclosed in Japanese Unexamined Patent Publication Hei 5-17523. A method of making use of doping in a slant direction is further disclosed in Japanese Unexamined Patent Publication Hei 7-307477.
Those techniques are, however, not always satisfactory because a width control of the LDD region cannot be done well or process steps thereof are more complicated. Objects of this invention are to provide an LDD TFT with an easily formed and precise structure, a manufacturing method for such an LDD TFT, and a TFT liquid crystal display device to which such an LDD TFT is applied.
A method of making a TFT of the invention includes a few steps. In the first step, a semiconductor layer is formed on one main surface of a substrate made essentially of a glass material. The semiconductor layer is then subject to a patterning step through which it is made island-like in configuration. The main substrate surface and the semiconductor layer are coated with first and second films of an insulator and a conductor, respectively. Another patterning step is applied to the second film to form a gate electrode on the substrate. The gate electrode is laminated with third and fourth films which are different from each other in film property. An anisotropic etching step is performed for the third and fourth films to leave the third film on the gate electrode and to form a side wall thereof. This method is further provided with a step of implanting ions into the semiconductor layer through the side wall.
Alternatively, a method of making a TFT of the present invention is performed by another series of steps. In this method, a gate electrode is formed on a main surface of a substrate made essentially of a glass material. A first film of an insulator is formed on the gate electrode in the next step. A semiconductor layer is then formed on the first film and a patterning step is further applied to make the same and the gate electrode island-like in configuration. A second film is formed on the semiconductor layer. A projected portion or prominence is made on the second film through another patterning step. Third and fourth films which are different from each other in film property are laminated to cover the prominence on the main surface of the substrate. An anisotropic etching step is performed for the third and fourth films to leave the former and to form around the prominence. The method also has a step of implanting ions into the semiconductor layer through the side wall.
The present invention provides TFTs manufactured by the methods set forth above.
The invention further provides a TFT liquid crystal display device for which TFT pixel switches or/and drivers are used.
The above-stated and other objects and technical features of the present invention will become apparent from the following description when taken with the accompanying drawings. It will be understood, however, that the drawings are for purposes of illustration and are not to be construed as defining the scope of the invention, reference being had for the latter purpose to the claims appended hereto.